The present invention relates generally to transversal filters, and more particularly, to a transversal filter allrate equalizer for use at intermediate frequency.
Transversal filter equalizers operating at baseband have been used extensively in high rate digital modems to improve data quality by compensating for amplitude and delay distortion. By way of example, a conventional adaptive baseband equalizer may be comprised of four transversal filters (two pair) and two adaptive control circuits. Two of the transversal filters (one pair) process the I samples and the remaining two transversal filters (the other pair) process the Q samples. Each adaptive control circuit is coupled to one transversal filter from each pair. The transversal filters operate in a frequency range from DC to a maximum frequency on the order of the data rate and the filter weights are real numbers, not complex numbers. The adaptive control circuits generate weights for the transversal filters that are required to minimize the distortion at the outputs. Many well-known algorithms may be used to accomplish this.
Transversal filter equalizers may be used with many PSK and QAM modulation schemes, including QPSK, MSK, 8PSK, and 16 QAM. The transversal filter equalizer is typically disposed between a quadrature demodulator and a data detector of a modem, for example. The distorted I or Q symbol waveform received from the data detector is processed by the transversal filter equalizer. For a typical symbol, the contribution of energy at sampling times used by a decision circuit in the equalizer other than its own sampling instant are zero. In other words, zero voltage crossovers for the symbol occur exactly at adjacent symbol sampling instants. If any energy exists during these adjacent sampling instants, then these adjacent bits may be detected in error, thus increasing the bit error rate. A corrected bit waveform formed after passing through a high quality adaptive baseband equalizer is such that the voltages at all sampling points, except for the main bit, are zero. Thus the adaptive baseband equalizer contributes significantly to the low error performance of a modem, for example.
Conventional equalizers are disclosed in the following U.S. patents and publication. U.S. Pat. No. 3,878,468 issued to Falconer et al. describes a conventional baseband equalizer. In this invention the base-band I and Q signals are recovered by sampling the IF and are processed in separate transversal filters. U.S. Pat. No. 3,974,449 issued to Falconer describes an improvement to the U.S. Pat. No. 3,878,468 invention by using several feed forward taps in addition to the transversal filter. U.S. Pat. No. 4,422,175 issued to Bingham et al. describes methods relating to a baseband equalizer. This patent illustrates methods of deriving the tap weights, not the transversal filter itself. U.S. Pat. No. 4,594,725 issued to Desperben et al. discloses a conventional transversal equalizer. U.S. Pat. No. 4,607,377 issued to Atobe et al. describes methods relating to deriving the tap weights. The implementation of the transversal filter is at baseband. A publication entitled "Jointly Adaptive Equalization and Carrier Recovery in Two-Dimensional Digital Communication Systems", by D. D. Falconer, AT&T, BSTJ, March 1979, pp. 317-333, describes a conventional baseband equalizer. In the circuit, described in this paper, the base-band I and Q signals are recovered by sampling the IF and are processed in separate transversal filters.
U.S. Pat. No. 4,475,211 issued to Mattis, Jr. et al. discloses a transversal equalizer that performs equalization at IF, demodulates the input signal and then generates the equalization coefficients at baseband from the demodulated data. The Mattis patent discloses an equalizer at IF, but uses whole symbol period delays and thus is not applicable to multirate or allrate operation. Also delay lines operating at IF produce not only a time delay for the modulation but also a phase shift for the carrier. This phase shift can be compensated for by adjusting the phases of the tap weights, W.sub.i. The Mattis patent does not deal with this phase shift problem, even though it does include an adaptive controller.
However, conventional equalizers are relatively complex, because they operate at baseband and the frequency response has to be uniform over many octaves, from DC, or almost DC, to at least the data symbol rate. Also the multipliers are many in number and very complex due to the aforementioned frequency response. Furthermore the delay paths generally must be hand tuned. Therefore, it is an objective of the present invention to provide for a transversal filter allrate equalizer that operates at intermediate frequency that overcomes some of the limitations of conventional equalizers.